Answer
Chorionic villi bring maternal and fetal blood vessels close together.
Work Step by Step
Chorionic villi of the embryo project into the uterine endometrium, and develop blood vessels which connect with the embryonic heart by way of umbilical arteries and vein; these blood vessels become folded in the umbilical stalk. The fetal and maternal blood come into close proximity, and are able to exchange nutrients wastes and gases ; however, under normal circumstances, the two blood systems do not mix.
